Chapter 7. Stabilizing Climate: Temperature Trends and Effects

Since 1970, the earth’s average temperature has risen by 0.7 degrees Celsius, or nearly 1.3 degrees Fahrenheit. Each decade the rise in temperature has been greater than in the preceding one. (See Figure 7–1.) Four of the six warmest years since recordkeeping began in 1880 have come in the last six years. Two of these, 2002 and 2003, were years in which, as just described, the major food-producing regions saw their crops wither in the presence of record or near-record temperatures. 14

As atmospheric concentrations of CO2 rise, so does the earth’s temperature. Since atmospheric CO2 permits sunlight to freely penetrate the earth’s atmosphere but restricts the radiation of heat back into space, it creates a “greenhouse effect.”

Atmospheric concentrations of CO2, estimated at 280 parts per million when the Industrial Revolution began, have been rising ever since people in Europe began burning coal. (See Figure 7–2.) They have risen every year since precise measurements began in 1959, making this one of the world’s most predictable environmental trends. As shown in Figure 7–2, atmospheric CO2 concentrations turned sharply upward around 1960. Roughly a decade later, around 1970, the temperature too began to climb; the rise since then is quite visible in Figure 7–1. Projections by the Intergovernmental Panel on Climate Change (IPCC) show temperatures rising during this century by 1.4–5.8 degrees Celsius. The accelerating rise in temperature in recent years appears to have the world headed toward the upper end of that projected range of increase. 15

Perhaps even more important than the average temperature rise is where the increase is likely to be concentrated. The warming will be greater over land than over the oceans, in the higher latitudes than in the equatorial regions, and in the interior of continents than in the coastal regions. One of the higher increases is expected to be in the interior of North America—an area that includes the grain-growing Great Plains of the United States and Canada and the U.S. Corn Belt, the very region that makes this continent the world’s breadbasket. 16

The earth’s rising temperature affects food security in many ways. Much of the world’s fresh water is stored in ice and snow in mountainous regions. These “reservoirs in the sky” supply water for irrigation. But the reservoirs are now shrinking. A modest rise in temperature of 1 degree Celsius in mountainous regions can substantially alter the precipitation mix between rain and snow, increasing rainfall and decreasing snowfall. This leads to more runoff during the rainy season and less snowmelt to feed rivers during the dry season, when farmers need irrigation water. 17

The melting glaciers and shrinking snowfields of the Himalayas are a concern to countries throughout Asia because this is where virtually all the major rivers in the region originate—the Indus, Ganges, Mekong, Yangtze, and Yellow. In Asia, where half the world’s people live and where irrigated agriculture looms large, any reduction in river flow during the summer directly affects food security. The prospect of diminished river flows during the dry season at a time when water tables are already falling in most Asian countries raises basic questions about food security in the region. 18

In addition to the direct effects of temperature on yield, higher temperatures mean more evaporation and thus more rainfall. Elevated temperatures can lead both to more extreme drought and to more severe flooding. Drought can be caused by below-normal rainfall or above-normal temperatures. Most often the two combine to create crop-withering droughts. Increased temperatures also mean more powerful, more destructive storms. 19

Higher temperatures can worsen or create new crop disease and insect problems. The combination of heat and humidity, which makes an ideal environment for many plant diseases, makes it almost impossible to produce wheat profitably in the tropics. Higher temperatures would simply expand the region that is inhospitable to wheat from the equator toward the higher latitudes. 20

One of the most serious long-term effects of climate change is rising sea level, which is driven both by the thermal expansion of the oceans as temperatures rise and by the melting of glaciers. The last IPCC report projected that sea level could rise by up to one meter during the current century, but papers published since then indicate that the melting is proceeding much faster than IPCC scientists had estimated. One study of glaciers in Alaska and Western Canada, for example, suggests that ice melting there is now raising sea level by 0.32 millimeters per year, more than double the 0.14 millimeters per year assumed by IPCC. 21

One of the major concerns among scientists today is the accelerated melting of the Greenland ice sheet. If the ice sheet on Greenland—an island three times the size of Texas—were to melt entirely, sea level would rise 7 meters (23 feet), inundating not only Asia’s rice-growing river deltas and floodplains but most of the world’s coastal cities as well. This kind of massive melting, even in the case of the most rapid warming scenario, would occur over centuries, however, not years. 22

The World Bank has published a map of Bangladesh, which shows that a 1-meter rise in sea level would inundate half of the country’s riceland. It would also displace some 40 million Bangladeshis. Where would these people go? Which countries would be willing to accept even a million refugees fleeing the effects of rising sea level? 23

A warmer earth means that agricultural zones in the northern hemisphere would move northward within Canada and Russia, for example, as the growing season lengthens. This assumes, of course, that there are high-quality soils that could sustain a productive agriculture in these regions. In Canada, however, the glaciated soils north of the Great Lakes cannot begin to match the productivity of the deep, fertile U.S. Corn Belt soils south of the Great Lakes. 24

One advantage of a longer growing season would be that the winter wheat belt could move northward, replacing the lower-yielding spring wheat now grown in the northernmost agricultural regions. This would affect primarily Canada and Russia, the leading producers of spring wheat. 25

On balance, however, agriculture would be a heavy loser if temperature continues to rise. The notion that the world’s farmers would be better off with more atmospheric CO2 and higher temperatures is a view based more on wishful thinking than on science. It may soon become apparent that the costs of climate change are unacceptably high.

15. Figure 7–2 from C. D. Keeling, T. P. Whorf, and the Carbon Dioxide Research Group, “Atmospheric Carbon Dioxide Record from Mauna Loa,” Scripps Institution of Oceanography, University of California, at cdiac.esd.ornl.gov/trends/trends.htm, updated June 2004; Intergovernmental Panel on Climate Change (IPCC), Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report (New York: Cambridge University Press, 2001).